In the past year or so there has been increasing interest, both nationally and internationally, in the development and use of severity scales to describe the impact of incidents and accidents at nuclear installations. The issues involved in developing such scales have been reviewed within the CEGB and the merits and disadvantages of various approaches analysed. The main points to emerge from this review are summarised as an input to the wider debate that is developing, in particular in an international framework, on the potential benefits and uses of severity scales.